Magnetic slip clutches employ an arrangement of magnetic members to provide a desired slip torque between input and output members. Magnetic slip clutches are more reliable than mechanical slip clutches, because there is less direct contact among the parts of a magnetic clutch as compared to a mechanical clutch, and thus the slip torque of a magnetic clutch is less likely to vary over time.
The "slip torque" of a magnetic slip clutch can be generally defined as the amount of torque that must be provided to one part of the clutch to cause it to rotate relative to another part of the clutch. If the clutch is incorporated in a cylindrical roller, for example, the outer surface of the roller rotates relative to a fixed shaft. However, the outer surface will not rotate relative to the shaft unless there is provided to the roller a torque greater than the slip torque of the clutch. If this torque is insufficient, the outer surface will remain rigid relative to the shaft and the roller will not rotate relative to the shaft. The slip torque is a function of the strength and relative positions of the magnetic members used in the clutch. Because there may be variations in the strength of magnetic members of a particular type, and because of manufacturing variations which cause differences in spacing among the magnetic members in each clutch, the slip torque among individual magnetic clutches of the same design may vary significantly.
Magnetic clutches have many applications in copiers and printers, but they are usually used in sheet feeding situations. "Sheets," as used in the specification and claims herein, may be either original documents to be copied, or copy sheets onto which images are subsequently copied or printed. The key problems of sheet feeding, as is well known in the art, include multifeeding (the passage of more than one sheet through a nip in the apparatus where only one sheet at a time is intended), crumpling, and jamming. Any number of prior art arrangements for obviating these common problems of sheet feeding devices rely on a predetermined relationship among such factors as the coefficients of friction between two sheets or between one sheet and a surface of the apparatus, and the slip torques of various rollers in the system. An unsuitable slip torque in any part of the system could defeat the purpose of such anti-misfeeding devices.
One solution to the problem of insuring a proper slip torque for a magnetic clutch in a sheet feeding or other system is to provide a magnetic clutch with an adjustable slip torque. In this way, a large number of clutches may be manufactured relatively inexpensively, and the "fine tuning" of the various individual clutches may be performed upon installation of each clutch in each particular machine. Indeed, it is conceivable that many clutches of the same manufactured type may be employed at a plurality of places in an apparatus, or even used in a variety of types of apparatus, the slip torque of each clutch being adjusted as necessary.
U.S. Pat. No. 1,862,267 to Honing discloses an adjustable magnetic slip clutch for use in a wire-winding machine. An inner clutch member attached to one portion of the clutch is axially movable within a cup-shaped second magnetic member, and the slip torque is variable by moving the members relative to each other to vary the amount of surface area between the two members. The two members are disposed in a self-regulator arrangement with a tensioner member which moves according to the tension of wire being wound on a spool attached to the clutch. U.S. Pat. No. 3,974,884 to Gidlund discloses a power wrench with a variable coupling between a driving member and a driven member. A clutch is formed by two intercoupled sleeves which are interlockable in preselected relative angular positions. U.S. Pat. No. 4,239,092 to Janson relates to a magnetic tensioning device comprising a rotatable outer housing and a magnetic assembly positioned in the interior of the outer housing. A space is provided between the outer housing and the magnetic assembly. A plurality of magnetic particles are positioned in the space between the magnetic assembly and the outer housing, held in place by flux from the magnetic assembly. U.S. Pat. No. 4,469,220 to Becker discloses a magnetic coupling assembly for driving a conveyor roll from a power drive. The conveyor roll includes a tube portion having a permanent magnet therein and a field face member secured in opposing relationship to the permanent magnet. The torque capacity of the roller is varied by varying the air gap between the permanent magnet and field face member.
It is an object of the present invention to provide a magnetic clutch with an adjustable slip torque which may be easily incorporated within the structure of a roller for use in a sheet feeding apparatus.
It is another object of the present invention to provide such a magnetic clutch which is simple to manufacture, simple to adjust, and resistant to drift in its selected slip torque.
It is another object of the present invention to provide an anti-misfeeding device for use in a paper-feeding apparatus, as would be used, for example, in an electrophotographic printer.
Other objects will appear hereinafter.